Close but yet far apart?

ok so im still friends with my ex and we were together for 5 tears and recently broke up. everytime i visit her to hang out she doeant mind us doing boyfriend girlfriend stuff to the point we get a hotel and have sex. but as soon as i go back to my state after a week or so we start getting distance. i know she doesn't have a guy becuase her parents monitor her and they like me so they dont mind me visiting her. im confused why its like this she's the one that left yet allows all this when i ask her to be my girlfriend she says no but yet lets this friends with benfits happen. any ideas why this is going on?